The following error happens "ajax.module: An unknown error has occurred." when a user clicks 'submit' more than once (which happens quite often).
This is often accompanied by "This content has been modified by another user." because it things someone else is saving the form as it's being edited.
The submit button needs to be deactivated when the user clicks submit or else this will happen every time. I believe this is also the cause of various similar error with this module.